The present invention relates to a zone melting process wherein an elongated charge of meltable material has, in a zone migrating in the direction of the longitudinal extent of the charge, a temperature above the melting point of the charge. After the migrating zone, in the direction of migration, the charge has a temperature at least below the melting point, and an electric current is passed through the charge in the longitudinal direction of the charge which is capable of heating the charge to a temperature above the melting point. The charge is cooled to a temperature below the melting point, at least in the region positioned after the migrating zone, in the direction of travel of the zone, according to German Patent Application P 21 25 726.1, the teachings of which are incorporated by reference herein.
From German Pat. No. 1,169,683 a process is known for zone melting a semiconductor rod without the use of a crucible or boat, wherein the molten zone is produced by a radiant energy source directed at right angles to the semiconductor rod and concentrically surrounding the rod. An approximately level course is obtained for the boundaries between the molten zone and the solid semiconductor material supporting it. Heating in the region of the molten zone is to be provided in a substantially linear form. Further, the heating of this region is to be to such an extent that this region is substantially smaller than the length of the molten zone. In brief, to achieve a short molten zone in the longitudinal direction, heating is to be carried out only in a short region. Additionally, heating of the semiconductor rod is provided by a current flowing through it in the longitudinal direction. In this way, it is intended to achieve a general, preferably uniform heating so that the temperature achieved by the heating is below the melting point. The object of this known process is to produce as level a boundary as possible between the liquid phase and the solid phase.
The older patent application P 21 25 726.1, proposes a zone melting process wherein an elongated charge of meltable material has, in a zone migrating in the direction of the longitudinal extent of the charge, a temperature above the melting point and after the migrating zone in the direction of migration, has a temperature at least below the melting point. The particular feature of that process is that the current flowing through the charge in the longitudinal direction is capable of heating the charge to a temperature above the melting point, and that the charge, at least in the region situated after the zone in the direction of travel of the zone, is cooled to a temperature below the melting point. In this way it is not necessary to introduce energy in the region of the melting zone for producing and maintaining the melting zone, as was the case with the process proposed in German Pat. No. 1,169,683. The current heats the charge in the region of the melting zone above the melting point, and, outside the melting zone, suitable cooling insures that the liquid phase merges into the solid phase or that the charge remains unmelted. With this process the boundary between the liquid phase and the solid phase is substantially a plane.
If the electrical resistance of the charge increases with increasing temperature or, on the solid charge, changing to the liquid phase, the voltage drop across this zone, and therefore the power produced, also rises. Therefore, it is very easy to bring about a stable condition by the cooling. But not all materials exhibit this resistance pattern, so that it becomes more complicated to maintain the necessary conditions.